Open loop DC control for a transimpedance feedback amplifier

ABSTRACT

A transimpedance amplifier having open-loop DC control is provided. The open-loop feedback control may provide a DC bias that is configurable based on the characteristics of an input device, such as, a photodiode or a magnetoresistor. The open-loop feedback control may provide quick recovery from voltage level variations and may provide stability for the amplifier.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 12/174,882 filed Jul. 17, 2008, which is a continuation of U.S. application Ser. No. 11/206,384 filed Aug. 17, 2005, now U.S. Pat. No. 7,449,958, which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

This invention relates to a transimpedance amplifier having open-loop DC control.

Transimpedance amplifiers, also commonly referred to as current-to-voltage converters, are frequently used to amplify input signals from sensors, such as, a photodiodes or a magnetoresistors. Transimpedance amplifiers provide high gain while also providing fast response times. The transimpedance amplifier must be properly DC biased to maintain the sensor biased at a proper DC operating level.

A feedback amplifier may be necessary to maintain the DC levels of the transimpedance amplifier at an appropriate voltage level. Closed-loop feedback amplifiers are often used to maintain the DC voltage levels, however, closed-loop feedback amplifiers may be prone to feedback delays or instability.

In view of the forgoing, it would be desirable to provide a transimpedance amplifier having open-loop DC control.

SUMMARY OF THE INVENTION

These and other objects of the invention are provided by a transimpedance feedback amplifier having open-loop DC control.

The open-loop feedback control may provide a DC bias that is configurable based on the characteristics of the input device. The open-loop feedback control may provide quick recovery from voltage level variations and may provide stability for the amplifier.

Further features of the invention, its nature and various advantages, will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified diagram of a transimpedance feedback amplifier having open-loop DC control in accordance with some aspects of the present invention.

FIG. 2 shows a simplified diagram of another transimpedance feedback amplifier having open-loop DC control in accordance with some aspects of the present invention.

FIG. 3 shows a more detailed simplified diagram of a transimpedance feedback amplifier having open-loop DC control in accordance with some aspects of the present invention.

FIG. 4 shows a simplified diagram of a differential transimpedance feedback amplifier having open-loop DC control in accordance with some aspects of the present invention.

DETAILED DESCRIPTION

FIG. 1 shows transimpedance feedback amplifier 100 with open-loop DC control in accordance with some aspects of the present invention. Transimpedance feedback amplifier 100 contains amplifier stage 110 and open-loop DC control stage 120. Amplifier stage 110 is AC coupled to V_(IN), the output of input device 105, which may be biased by bias current I_(BIAS). Input device 105 may be, for example, a photodiode or a magnetoresistor. AC coupling input capacitor C_(IN) is connected to V_(IN) and amplifier 107. The output of amplifier 107 is connected to series capacitor C₁. In some embodiments of the present invention, C₁ may be a small capacitor (e.g., approximately five picofarads). The gate of transistor M1 is connected to the output of series capacitor C₁. The drain of transistor M₁ is connected to the power supply V_(DD). Current source I_(M1) is connected between the source of transistor M1 and ground. The source of transistor M₁ also provides DC feedback to input node V_(IN) through feedback resistor R_(F). In the present embodiment of the invention, the output of transimpedance feedback amplifier 100, V_(OUT) may be taken from the source of transistor M₁. Alternatively, the output voltage may be taken from node V₁. While in the present embodiment MOS transistors are used, it should be understood that BJT transistors may also be used in accordance with the present invention.

During operation of transimpedance feedback amplifier 100, amplifier stage 110 amplifies the high-frequency components of the input signal V_(IN). The low-frequency cutoff of the amplifier is set by the capacitances of capacitors C_(IN) and C₁, the input impedance of amplifier 107, and the resistance of resistor R₂.

Open-loop DC control stage 120 is DC coupled to the gate voltage of transistor M1. Open-loop DC control stage 120 contains resistor R₂, transistor M2, voltage buffer 122, switch 124, and variable current source I_(M2). The gate-source voltages of transistors M1 and M2 may be substantially equal when the magnitudes of I_(M1) and M1 are in proportion to the magnitudes I_(M2) and M2. For example, the magnitudes of I_(M1) and M1 may be a tenth of the magnitude of I_(M2) and M2. Because open-loop DC control stage 120 does not receive feedback from amplifier stage 110, DC control stage 120 may be very stable and may recover quickly from variations in the DC voltage level.

After transimpedance feedback amplifier 100 is properly biased, the DC voltage level of V_(IN) is approximately equal to the DC voltage level of V_(OUT) and the current flowing through feedback resistor R_(F) is minimized. The current generated by current source I_(M2) may be adjusted to change the DC voltage level of V_(OUT) in order to set V_(OUT) approximately equal to any suitable DC voltage level of V_(IN).

Voltage buffer 122 may allow voltage V₂ to drive capacitor C₁ through resistor R₂. During fast switching of transimpedance feedback amplifier 100 a control signal (not shown) may cause switch 124 to short-circuit resistor R₂, which may allow voltage buffer 122 to directly drive capacitor C₁ to quickly charge the capacitor. For example, when transimpedance amplifier 100 is turned on, capacitor C₁ needs to be charged to an appropriate DC voltage level before the circuit can operate properly.

FIG. 2 shows transimpedance feedback amplifier 200 in accordance with some aspects of the present invention. Transimpedance feedback amplifier 200 operates in a similar manner as transimpedance feedback amplifier 100. Transimpedance feedback amplifier 200 uses constant current source I_(M2′) in addition to programmable offset amplifier 230, within open-loop DC control stage 220 to adjust the DC voltage level instead of using variable current source I_(M2). Programmable offset amplifier 230 may be set to provide and offset voltage approximately equal to the DC voltage level of V_(IN). Programmable offset amplifier 230 allows V_(OUT) to be set to approximately the same voltage level as V_(IN).

FIG. 3 shows transimpedance feedback amplifier 300 including a more detailed embodiment of programmable offset amplifier 230. Programmable offset amplifier 230 contains an amplifier, transistor M3, resistors R_(A1) and R_(A2), and variable current source I_(M3). The offset voltage of programmable offset amplifier 230 may be adjusted by variable current source I_(M3). Adjusting variable current source I_(M3) based on the characteristics of input device 105 may permit open-loop DC control stage 220 to maintain proper DC bias of transimpedance feedback amplifier 200.

FIG. 4 shows differential transimpedance feedback amplifier 400 in accordance with some aspects of the present invention. Differential transimpedance feedback amplifier 400 is a differential version of transimpedance feedback amplifier 200 and operates in a substantially similar manner as transimpedance feedback amplifier 200. Differential transimpedance feedback amplifier 400 may be powered by positive and negative power supplies V_(DD) and V_(SS) and may provide differential outputs V_(OP) and V_(ON) using DC bias voltages V_(1P) and V_(1N). Differential transimpedance feedback amplifier 400 also includes current sources I_(BIASP), I_(BIASN), I_(M1P), I_(M1N), I_(M3), I_(B1), resistors R_(F1), R_(F2), R_(2P), and R_(2N), capacitors C_(IN1), C_(IN2), C₁, and C₂, buffers 122P and 122N, and transistors 124P and 124N, which all operate in a substantially similar manner to their counterparts in transimpedance feedback amplifier 200.

Thus it is seen that a transimpedance feedback amplifier having open-loop DC control has been provided. It will be understood that the foregoing is only illustrative of the principles of the invention, and that the invention can be practiced by other than the described embodiments, which are presented for purposes of illustration and not of limitation, and the present invention is limited only by the claims which follow. 

1. Control circuit for a transimpedance feedback amplifier, comprising: an open-loop DC control circuit in DC communication with a transimpedance feedback amplifier and that provides a DC bias voltage to the transimpedance feedback amplifier, wherein the DC bias voltage is adjusted using a variable current source.
 2. The control circuit of claim 1 wherein the transimpedance feedback amplifier comprises a differential amplifier and the open-loop DC control circuit provides differential DC bias voltages to the transimpedance feedback amplifier.
 3. The control circuit of claim 1 wherein the transimpedance feedback amplifier comprises an amplifier current that is proportional to a current of the variable current source.
 4. The control circuit of claim 1 further comprising a resistor and a bypass switch, each coupled between the open-loop DC control circuit and the transimpedance feedback amplifier, wherein the bypass switch is operative to bypass the resistor.
 5. The control circuit of claim 1 wherein the DC bias voltage is adjusted using a variable offset amplifier and wherein the variable offset amplifier is adjusted using the variable current source.
 6. The control circuit of claim 1 further comprising an input device in AC communication with the transimpedance feedback amplifier.
 7. The control circuit of claim 6 wherein the DC bias voltage is substantially equal to a DC voltage of the input device.
 8. The control circuit of claim 6 wherein the input device is selected from a group consisting of a photodiode and a magnetoresistor.
 9. A method for providing a control circuit for a transimpedance feedback amplifier, comprising: DC coupling an open-loop DC control circuit to the transimpedance feedback amplifier; generating a DC bias voltage using the open-loop DC control circuit; providing the DC bias voltage to the transimpedance feedback amplifier; and varying the DC bias voltage of the open-loop DC control circuit using a variable current source.
 10. The method of claim 9 wherein the transimpedance feedback amplifier comprises a differential amplifier and the open-loop DC control circuit provides differential DC bias voltages to the transimpedance feedback amplifier.
 11. The method of claim 9 wherein the transimpedance feedback amplifier comprises an amplifier current that is proportional to a current of the variable current source.
 12. The method of claim 9 further comprising bypassing a resistor coupled between the open-loop DC control circuit and the transimpedance feedback amplifier using a bypass switch.
 13. The method of claim 9 wherein using the variable current source to vary the DC bias voltage comprises varying a variable offset amplifier in the open-loop DC control circuit.
 14. The method of claim 9 further comprising AC coupling an input device to the transimpedance feedback amplifier.
 15. The method of claim 14 wherein the DC bias voltage is substantially equal to a DC voltage of the input device.
 16. The method of claim 14 wherein the input device is selected from a group consisting of a photodiode and a magnetoresistor. 